Ionization source



Jan. 3l, 1956 M. G. INGHRAM Erm. 2,733,343

IoNIzATIoN SOURCE Filed NOV. 24, 1955 2 Sheets-Sheet 2 United StatesPatent IONIZATINSOURCE Application November 24, `1953,-SerialNo.`394,`521

.8 Claims. (Cl. .Z50-41.9)

The present invention relates to .mass spectrometers, iandparticularlyAto4 sources of positive 'ion-s vsuitable for .use in massspectrometers.

. `Effhe-.use fof surface ionization sources, as distinguishedfronrelectron bombardment `sources and arc discharge ion sources, has ygreatly .increased in recent years. In simplest form, a :surfaceionization Vsource consists of a single .filament on `to which`anraliquoteof the sample to be analyzed, is placed, vand `by properselection of-the temper- .aturefofthelilamenn molecules and-atoms areevaporated .aslions .from the source. The eliiciency `of ionization ofsuch .a `.source .is .given by the .following equation:

isthe vratio of the chargedto the uncharged component, eis vtheelectronic charge, I is the work function of the evaporating surface, Iis the ionization potential of the evaporating component, K is theBoltzmann constant, and `Tis the absolute temperature. Such an ionsource is disclosed in the patent application of Mark G.Inghram andDavid C. Hess, 'Serial No. 306,844, filedAugust 28, 1952, entitled IonSource,.now Patent No. 2,710,354, issued June'7,`195,5.

.Several `ditliculties have been encountered in the use 4ofsurface'ionization sources, such as described above. First, such surfaceionization sources exhibit low ionization efficiency for substanceswhichphave ionization ,potentials which are greater than the workfunction of the evaporating surface and which evaporate atlow temper-Yatures. Low vionization eiiciency results under theseconditionsibecause the evaporation rate of the' material `and the'temperature at Ywhich evaporation .occurs cannot be variedindependently. The second difficulty encountered with such surfaceionization sources is Vthat the molecular form of -the evaporatedconstituent cannot be controlled, since the molecularform is determinedessentially Vonly by the temperature in this type of source and thetemperature must generally be selected topgive the maximum `evaporationrate. A third difficulty with surface ionization sources of this type isthat relatively high 'backgrounds occur by surface ionization ofhydrocarbons where the aliquot evaporates at a low temperature. It

is thus clear that the interdependence of the temperature andevaporation rate lin such surface ionization sources isa deterrent totheir use.

The present invention provides a surface ionization source in which theevaporationrate of the material to `he analyzed Vis independent of thetemperature at'which the Imaterial is evaporated, thereby alleviatingthe diiiiyculties which have yheretofore limited the use ofsuch surfaceVionization sources. The present 'invention therefore .provides :an ionsource which Vwill produce ionized .atomsfofelements'which heretoforehave been ionized by ice surface :ionization only in the form ofmolecules. The man skilled in the art will readily perceive many `otheruses and advantages of the present invention. A more completeunderstanding of the present invention may be had from a further readingof the disclosure, particularly when viewed in the light of thedrawings, in which:

Figure 1 is a sectional view of a mass spectrometer incorporating a.surface ionization source constructed `according to the teachings ofthe present invention;

.Figure 2 is-.a Vsectional view of the surface ionization sourceshown inFigure l taken along the line 2-2 indicated lin Figure 3;

Figure 3 is a sectional view taken along line 3-3 of Figure 2;

Figure 4 is a vertical sectional view through the ion source taken alongthe line 4-4 of Figure 3; .and

Figure 5 is a -sectional view taken along line 5-5 rof Figure 4.

As illustrated in Figure .1, the mass spectrometer is provided with acasing 10 in which a vacuum is maintained. The casing 10 is generallyY-shaped and is provided with a magnet 12 at the junction of the twolegs of the Y. An ion source 14, which will be described .in detailhereafter, is disposed within one of the legs of the casing 10 near theend thereof, and a collector 16 is .disposed adjacent to the end oftheother leg of the casing .10. Mass spectrometers of the type heregenerally .described are well known in the art, the copending patentapplication of Alfred O.C. Nier and Mark G. Inghram, .Serial No.554,965, led September 20, 1944, entitled Mass Spectrometer, now PatentNumber 2,551,544, issued May .1, 1951, discloses in detail a massspectrom- =eter of .the type generally shown in Figure 1. Y

The ion source 14, as illustrated in detail in Figures `2 .through 5, isprovided with four parallel rectangularl-y .positioned posts 17 whichsupport the other elementsof xthe tion source. The posts 17 extendbetween a vfilament shield plate 18 anda collimating slit plate 20. Afilament supporting member 22 is attached to the filament shield ,plate-18and supports an ionization filament 24 and two .evaporation filaments26 and 28.

The filament support member 22 has a generally U- .shaped central,portion 30 which is provided with flanges 32 .to facilitate .attachingthe member 22 to the filament shield plate 18. Two rectangular bars 34and 36 are Vattached to the supporting member 22 and span the openportion of the central portion 30 thereof parallel to the filamentshield plate 18. A pair of electrically conduct` 'ing'.posts 38 `extendthrough the bar 36 and are insulated therefrom .by insulators 42, theevaporation filament 26 being mechanicallyand electrically connected tothe posts .38. In like manner, a pair of electrically conducting posts40 .extend through the bar 34 and are insulated vtherefrom by insulator44, and support the evaporation V"filament 28. The ionization filament24 is likewise elec- 'ttrically connected to aA pair of supporting posts46 which extend `through insulators 48 in the central' portion 30 ofthe'tlilament support 22.

The filament shield plate 18 is provided with an aperture 49 ywhich isvgenerally shaped in the form of an H. The ionizationfilament 24 ispositioned in the cross bar of the V:H and centrally thereof. The twoevaporation filaments 26 and 28 are Vmounted parallel to the ionizationfilament 24 and extend from the cross bar of the H along .the legsthereof to their respective posts 38 and 40. The evaporation filaments26 and 28 are also positioned on the side .of the'filament shield plate18 opposite tothe support member`22, `so that .the evaporation filaments26 and 28 are positioned between the ionization filament 24 and thecollimating slit plate 20.

A number of slit plaies are mounted between the filament-shield plate 18and the collimating slit plate 20, these are in order of position fromthe filament shield plate 18, a defocussing slit plate 50, adiscriminating slit plate 52, a pair of focussing slit plates 54a and54h a collimating slit plate 56, and a pair of beam centering plates 58aand 58b. The collimating slit plate 20 is provided with a slit 60disposed centrally thereof and generally aligned with the cross bar ofthe H-shaped aperture 49 in the filament shield plate 18. Each of theplates 50, 52, and 56, which are disposed between the filament shieldplate 18 and the collimating slit plate 20, are provided with slits 70,72 and 76, respectively, which are aligned with the cross bar of theH-shaped aperture 49 in the filament shield plate 18 and the slit 60 ofthe collimating slit plate 20. Plates 54a and 54h are spaced from eachother forming a slit 74 therebetween, and beam centering plates 58a and58b are also spaced to form slit 78, slits 74 and 78 aligning with theother slits 70, 72, 76 and 60.

In a particular construction of the surface ionization source which willbe describe-d throughout this disclosure, the filaments 24, 26 and 28are constructed in the form of fiat ribbons of tungsten, the ribbonbeing 0.001 by 0.03 inch. The evaporation filaments 26 and 28 aredisposed as close to the ionization filament 24 as possible, the twoevaporation filaments 26 and 28 being mounted between the ionizationfilament 24 and the defocussing slit 70, so that the materialsevaporated from the filaments 26 and 28 will fall upon the surface ofthe ionization filament 24 which confronts the slits 70, 72, 74, 76, 78and 60. In this construction, the evaporation filaments 26 and 28 arespaced from the filament 24 by gaps of not more than 0.01 inch, and inany construction the evaporation filament should not be spaced from theionization filament by an average gap exceeding twice the width of ,theionization filament confronting the evaporation filament.

The length of the cross bar of the H-shaped aperture 49 is approximatelythree-eighths of an inch, while the total length of the two legs of theH-shaped aperture 49 is approximately five-eighths of an inch, thediameter of the filament shield plate 18, the plates 50, 52, 56 and 20being the combined halves of plates 54a and 5417, and 58a and 58b beingapproximately one and one-half inches each. The defocussing slit plate50 is positioned approximately 0.120 inch from the filament shield plate18; the discriminating slit plate 52 is positioned approximately 0.120inch from the defocussing slit plate 50; the focussing slit plates 54aand 54b are positioned approximately 0.240 inch from the discriminatingslit plate 52; the collimating slit plate 56 is positioned approximately0.160 inch from the focussing plates 54a and 54b; the beam centeringplates 58a and 58b are positioned approximately 0.160 inch from thecollimating slit plate 56; and the collimating slit plate is positionedapproximately 0.320 inch from the beam centering plates 58a and 58b. Theaperture 49 in the filament shield plate 18 which is in the form of an Hs approximately one-eighth inch wide for both the cross arm of the H andthe two legs thereof; the slit 70 in the defocussing slit plate 50 isapproximately 0.060 inch; the slit 72 in the discriminating plate 52 isapproximately 0.030 inch; the slit 74 between the focussing slit plates54a and 54h is approximately 0.080 inch; the slit 76 in the collimatingslit plate 56 is approximately 0.008 inch; the slit 78 separating beamcentering plates 58a and 58b is approximately 0.008 inch; and the slit60 in the collimating slit plate 20 is also approximately 0.008 inch.

The above dimensions are suitable for use with the following voltages:collimating slit plate 20, zero volts; beam centering plates 58a and 58baverage approximately +1000 volts, the voltage on these plates beingadjustable for purposes of centering the ion beam; the collimating slitplate 56, +1000 volts; the focussing slit plates 54a and 54h average+3500 volts, the potentials on these plates again being varied relativeto each other for centering purposes; the discriminating slit plate 52,+4580 volts; the

4 defocussing slit plate 50, +4980 volts; and the filament shield plate18, +5000 volts.

The temperatures at which the filaments operate are also important tothe production of ions. With the above described ion source, evaporationfilaments 26 and 28 may be coated with gadolinium oxide (GdzOz).Evaporation filaments 26 and 28 are then operated at a temperature ofl250 K., and ionization filament 24 is operated at 2500 K. With thisconstruction, it has been found that the ratio of the positively chargedcomponents evaporated from this source to the uncharged component isabout 3 times 10-5, whereas a single filament surface ionization source,such as previously known to the art, produces a ratio of approximately10-9. Also, such a single filament surface ionization source producesgadolinium ions mostly as GdO+, while the multiple filament ionizationsource here disclosed produces gadolinium ions in the form Gdr.

If uranium in the form of uranium oxide (U03) is disposed uponevaporation filaments 26 and 28, and ionization filament 24 is operatedat 2700" K. while evaporation filaments 26 and 28 are operated at 1250K., then the ion source disclosed, voltages and dimensions being maintained the same as previously, will produce Ur ions. An ion source witha single filament, as previously known, produces uranium ions primarilyof the form UOz+. Also, evaporation filaments 26 and 28 may be coatedwith an oxide of nickel such as NiO, and will produce positive ions ofnickel with evaporation filaments 26 and 28 operating at a temperatureof 850 K. and the ionization filament operating at a temperature of 2500K., the voltages and spacings being the same as previously disclosed,where the single filament ionization source is not known to producepositive ions of nickel except due to tertiary emissions.

As shown in Figure 1, the slit plates of the ion source are maintainedat the proper voltages by means of conventional power sources,illustrated as batteries. Batteries 80 and 82 are connected between thecollimating slit plate 20 and the beam centering plates 58a and 58b.respectively. Battery 84 is connected between the collimating slit plate20 and the collimating slit plate 56, batteries 86 and 88 are connectedbetween the collimating slit plate 20 and the focussing slit plates 54aand 54b, respectively. Battery 90 is connected between the collimatingslit plate 20 and the discriminating slit plate 52. Battery 92 isconnected between the collimating slit plate 20 and the defocussing slitplate 50. Also, battery 94 is connected between the collimating slitplate 20 and the filament shield plate 18. It has been found that a p0'tential of not more than 200 volts negative relative to the filamentshield plate 18 must be applied to the defocussing slit plate 50 and thediscriminating slit plate 52 in order to assure collimation of ionsionized by the ionization filament 24 alone. In addition, the batteries96, 98 and 100 are used to supply the filament currents for filaments24, 26 and 28, respectively.

In operating the multiple filament surface ionization source heredisclosed, the material to be ionized is placed upon evaporationfilaments 26 and 28, and the temperature of these filaments is selectedto produce the optimum evaporation rate for the material placed thereon.The evaporation filaments 26 and 28 may be operated at whatevertemperature is determined to be most efiicient for evaporation of thematerial by limiting the current flowing through the battery-filamentcircuits. Also, it is not necessary that two filaments be coated withthe material to be ionized, since one of the filaments 26 or 28 could beeliminated, the two filaments having been provided primarily forelectrical symmetry. In this manner, ions are emitted from theevaporation filaments 26 and 28; however, as stated above, the emittedions are often in the form of molecules of the metal to be ionized. Aproportion of the emitted molecules will strike the ionization filament24 which is operated at a very much higher temperature. The temperatureof the ionization filament 24 is `Vgenerally Las -high as possibleconsistent'with reasonable sor-I) 1 1 n+T2 ei H+T1- where T1 and T2 arethe temperatures of the ionization filaments in both sources, e is theelectronic charge, I is the work function of the evaporating surface, Iis the ionization potential of the evaporating component, K is theBoltzmann constant.

It is important that neither of the evaporation filaments 26 nor 28 isfocussed upon the slit 6G in the collimating slit plate 20, since theevaporation products from these filaments are of the same purity asthose from filament 24 and are not to directly constitute the ion beam.For this reason, the defocussing and discriminating slit plates 50 and52 are provided and are maintained at a potential slightly negativerelative to the filament shield plate 18.

From the foregoing disclosure of the invention, the man skilled in theart will readily devise many modifications and applications for thedevice here disclosed within the scope of the invention. Hence, it isintended that the invention be not limited by any specific illustrationset forth herein, but rather only by the appended claims.

What is claimed is:

1. A source of ions comprising, in combination, an ionization lament, anevaporation filament adapted to be coated with the material to beionized, means to mount said filaments in adjacent relationship, saidfilaments being spaced by an average distance equal to not more thantwice the width of the ionization filament confronting the evaporationfilament.

2. A source of ions comprising, in combination, an evacuated housing, anionization filament, an evaporation filament adapted to be coated withthe material to be ionized, means mounting said filaments within thehousing in adjacent relationship, said filaments being spaced by anaverage distance equal to not more than twice the width of theionization filament confronting the evaporation filament.

3. A source of ions comprising, in combination, an evacuated housing, anionization filament, an evaporation filament adapted to be coated withthe material to be ionized, means mounting said filaments within thehousing in adjacent relationship, said filaments being spaced by anaverage distance equal to not more than twice the width of theionization filament confronting the evaporation filament, means tooperate the evaporation filament at a temperature producing evaporationof the material to be ionized, means to operate the ionization filamentat a temperature producing ionization of the molecules of the materialto be ionized, and means to impel the ions produced from the ionizationfilament.

4. A source of ions comprising, in combination, an evacuated housing, anionization filament, a pair of evaporation filaments adapted to becoated with the material to be ionized, means disposed within thehousing to mount said filaments in adjacent relationship, each of theevaporation filaments being spaced from the ionization filament by anaverage distance equal to not more than twice the width of theconfronting ionization filament, means to operate the evaporationfilaments at a temperature producing maximum evaporation of the materialto be ionized, means to operate the ionization filament at a temperatureof at least 2500 K., and means to impel the ions produced from theionization filament.

(sv I 5. A source of ions comprising, in combination, an ,ionizationfilament, an evaporation filament adapted .to .be `coated with the:material to lbe ionized, means to mount said filaments in adjacentrelationship, said filaments being spaced by an `average. distance equal'to not more than twice the width of the ionization filament confrontingthe evaporation filament, and means to maintain the temperature of theionization filament above that of the evaporation filament.

6. A source of ions comprising, in combination, an evacuated housing, anionization lament, an evaporation filament adapted to be coated with thematerial to be ionized, means mounting said filaments within the housingin adjacent relationship, said filaments being spaced by an averagedistance equal to not more than twice the width of the ionizationfilament confronting the evaporation filament, and means to impel theions produced from the ionization filament including a filament shieldplate and at least two slit plates, the filament shield plate having anaperture disposed therein positioned adjacent to the ionizationfilament, and the other two slit plates having slits aligned with theionization filament, the slit plate disposed between the filament shieldplate and the other slit plate being maintained at a potential not morethan 200 volts negative relative to the filament shield plate, and theother slit plate being maintained at a potential more negative than thefirst slit plate.

7. A source of ions comprising, in combination, an evacuated housing, anionization filament, an evaporation filament adapted to be coated withthe material to be ionized, means mounting said filaments within thehousing in adjacent relationship, said filaments being spaced by anaverage distance equal to not more than twice the width of theionization filament confronting the evaporationfilament, means tooperate the evaporation filament at a temperature producing evaporationof the material to be ionized, means to operate the ionization filamentat a greater temperature than the evaporation filament, and means toimpel the ions produced from the ionization filament including afilament shield plate having an aperture therein, the ionizationfilament being disposed within the aperture of the filament shieldplate, and at least two slit plates, the slit plates having slitsaligned with the ionization filament, and means to maintain the slitplate adjacent to the filament shield plate at a potential negative tothe filament shield plate by not more than 200 volts, and means tomaintain the other slit plate at a potential more negative than thefirst slit plate relative to the filament shield plate.

8. A source of ions comprising, in combination, an evacuated housing, anionization filament, a pair of evaporation filaments adapted to becoated with the material to be ionized, means disposed within thehousing to mount said filaments in adjacent relationship, each of theevaporation filaments being spaced from the ionization filament by anaverage distance equal to not more than twice the width of theconfronting ionization filament, means to operate the evaporationfilaments at a temperature producing maximum evaporation of the materialto be ionized, means to operate the ionization filaments at atemperature of at least 2500" K., and means to impel the ions producedfrom the ionization filament including a filament shield plate having anaperture therein including a portion forming a slit, the ionizationfilament being disposed within said slit portion and the evaporationfilaments being disposed adjacent thereto, a defocussing slit platedisposed adjacent to the filament shield plate, a discriminating slitplate disposed adjacent to the defocussing slit plate, a pair offocussing slit plates disposed adjacent to the discriminating slitplate, a collimating slit plate disposed adjacent to the defocussingslit plates, a pair of beam centering plates disposed adjacent to thecollimating slit plate, and a second collimating slit plate disposedadjacent to the beam centering plates, the defocussing slit plate,discriminating slit plate,

collimating slit plate, and second collimating slit plate being providedwith aligned slits and the focussing slit plates and beam centering slitplates being spaced from each other to form slits aligned with the slitsin the slit plates, means to apply a large positive potential on thefilament shield plate relative to the second collimating slit plate, andmeans to apply a potential of not more than 200 volts negative withrespect to the lament shield plate on the defocussing slit plate and onthe discriminating slit plate.

No references cited

1. A SOURCE OF IONS COMPRISING IN COMBINATION, AN IONIZATION FILAMENT,AN EVAPORATION FILAMENT ADAPTED TO BE COATED WITH THE MATERIAL TO BEIONIZED, MEANS TO MOUNT SAID FILAMENTS IN ADJACENT RELATIONSHIP, SAIDFILAMENTS BEING SPACED BY AN AVERAGE DISTANCE EQUAL TO NOT MORE THANTWICE THE WIDTH OF THE IONIZATION FILAMENT CONFRONTING EVAPORATIONFILAMENT.